Lyme borreliosis is the most common vectorborne infection in the United States [S. W. Barthold, et al., "An Animal Model For Lyme Arthritis", Ann. N.Y. Acad. Sci., 539, pp. 264-73 (1988)]. It has been reported in every continent except Antarctica. The clinical hallmark of Lyme Disease is an early expanding skin lesion known as erythema migrans, which may be followed weeks to months later by neurologic, cardiac, and joint abnormalities.
The causative agent of Lyme disease is a recently recognized spirochete known as Borrelia burgdorferi, transmitted primarily by ixodes ticks that are part of the Ixodes ricinus complex. B. burgdorferi has also been shown to be carried in other species of ticks and in mosquitoes and deer flies, but it appears that only ticks of the I. ricinus complex are able to transmit the disease to humans.
Lyme disease generally occurs in three stages. Stage one involves localized skin lesions (erythema migrans) from which the spirochete is cultured more readily than at any other time during infection [B. W. Berger et al., "Isolation And Characterization Of The Lyme Disease Spirochete From The Skin Of Patients With Erythema Chronicum Migrans", J. Am. Acad. Dermatol., 3, pp. 444-49 (1985)]. Flu-like or meningitis-like symptoms are common at this time. Stage two occurs within days or weeks, and involves spread of the spirochete through the patient's blood or lymph to many different sites in the body including the brain and joints. Varied symptoms of this disseminated infection occur in the skin, nervous system, and musculoskeletal system, although they are typically intermittent. Stage three, or late infection, is defined as persistent infection, and can be severely disabling. Chronic arthritis, and syndromes of the central and peripheral nervous system appear during this stage, as a result of the ongoing infection and perhaps a resulting auto-immune disease [R. Martin et al., "Borrelia burgdorferi--Specific And Autoreactive T-Cell Lines From Cerebrospinal Fluid In Lyme Radiculomyelitis", Ann Neurol., 24, pp. 509-16 (1988)].
B. burgdorferi is much easier to culture from the tick than from humans, therefore at present, Lyme disease is diagnosed primarily by serology. The enzyme-linked immunosorbent assay (ELISA) is one method of detection, using sonicated whole spirochetes as the antigen [J. E. Craft et al., "The Antibody Response In Lyme Disease: Evaluation Of Diagnostic Tests", J. Infect. Dis., 149, pp. 789-95 (1984)]. However, serologic testing is not yet standardized, and results may vary between laboratories and commercial kits, causing false negative and, more commonly, false positive results. In addition, the disease often goes unrecognized, as the ticks are small and easy to miss, and the characteristic rash only occurs in 60-80% of cases and may be misinterpreted when it does occur.
At present, all stages of Lyme disease are treated with antibiotics. Treatment of early disease is usually effective, however the cardiac, arthritic, and nervous system disorders associated with the later stages often do not respond to therapy [A. C. Steere, "Lyme Disease", New Eng. J. Med., 321, pp. 586-96 (1989).
Two lines of evidence suggest that the host immune response to specific antigens of B. burgdorferi may be partially responsible for the pathogenicity of Lyme disease. First, patients treated with corticosteroids (which suppress the immune system) show improvement of their symptoms [A. C. Steere et al., "Lyme Carditis: Cardiac Abnormalities Of Lyme Disease", Ann. Intern. Med., 93, pp. 8-16 (1980)]. Second, some patients that do not respond to antibiotics appear to manifest an autoimmune disorder initiated by infection with B. burgdorferi.
Like Treponema pallidum, which causes syphilis, and leptospirae, which cause an infectious jaundice, Borrelia belong to the eubacterial phylum of spirochetes [A. G. Barbour and S. F. Hayes, "Biology Of Borrelia Species", Microbiol. Rev., 50, pp. 381-400 (1986)]. Borrelia burgdorferi have a protoplasmic cylinder that is surrounded by a cell membrane, then by flagella, and then by an outer membrane. Embedded in the outer membrane are two major proteins, a 31 kd outer-surface protein A (OspA) [A. G. Barbour et al., "Lyme Disease Spirochetes And Ixodid Tick Spirochetes Share A Common Surface Antigenic Determinant Defined By A Monoclonal Antibody", Infect. Immun., 41, pp. 795-804 (1983); J. L. Benach et al., "A Murine IgM Monoclonal Antibody Binds An Antigenic Determinant In Outer Surface Protein A, An Immunodominant Basic Protein Of The Lyme Disease Spirochete", J. Immunol., 140, pp. 265-72 (1988)] and a 34 kd outer surface protein B (OspB) [A. G. Barbour et al., "Variation In A Major Surface Protein Of Lyme Disease Spirochetes", Infect. Immun., 45, pp. 94-100 (1984)]. The two proteins have been shown to vary from different isolates or from different passages of the same isolate as determined by their molecular weights and reactivity with monoclonal antibodies. In addition, OspB may not be produced at all in culture [T. G. Schwan et al., "Changes In Infectivity And Plasmid Profile Of The Lyme Disease Spirochete, Borrelia Burgdorferi, As A Result Of In Vitro Cultivation", Infect. Immun., 56, pp. 1831-36 (1988)].
Early in human infection, antibodies are generated primarily against a 41 kd flagella-associated antigen. Later on, high titer antibodies to both OspA and OspB appear [J. E. Craft et al., "Antigens Of Borrelia Burgdorferi Recognized During Lyme Disease: Appearance Of A New Immunoglobulin M Response And Expansion Of The Immunoglobulin G Response Late In The Illness", J. Clin. Invest., 78, pp. 934-39 (1986)]. However, this humoral immune response is generally not sufficient to clear the system of the infective agent in experimentally infected laboratory rats. [K. D. Moody et al., "Experimental Chronic Lyme Borreliosis In Lewis Rats", Am. J. Trop. Med. Hyg. in press (1990)]. In addition, humans have been shown to be persistently infected for months or years. It has thus been suggested that the spirochete may be able to sequester itself in certain intracellular sites where it remains unavailable to circulating antibody molecules.
Development of a laboratory model for Lyme disease has proved elusive. Several groups have found spirochetemia in rabbits, Peromyscus mice, and Syrian hamsters after inoculation with B. burgdorferi, but no other manifestations of Lyme disease have been found. [W. Burgdorferi, "The New Zealand White Rabbit: An Experimental Host For Infecting Ticks With Lyme Disease Spirochetes", Yale J. Biol. Med., 57, pp. 609-12 (1984); A. N. Kornblatt et al., "Experimental Lyme Disease In Rabbits: Spirochetes Found In Erythema Migrans And Blood", Infect. Immun., 46, pp. 220-23 (1984); A. N. Kornblatt et al., "Infection In Rabbits With The Lyme Disease Spirochete", Yale J. Biol. Med., 57, pp. 613-14 (1984); J. L. Benach et al., "Experimental Transmission Of The Lyme Disease Spirochete To Rabbits", J. Infect. Dis., 150, pp. 786-87 (1984); J. G. Donahue et al., "Reservoir Competence Of White-Footed Mice For Lyme Disease Spirochetes", Am. J. Trop. Med. Hyg., 36, pp. 92-96 (1987); E. C. Burgess et al., "Experimental Inoculation Of Peromyscus spp. With Borrelia Burgdorferi: Evidence Of Contact Transmission", Am. J. Trop. Med. Hyg., 35, pp. 355-59 (1986); P. H. Duray and R. C. Johnson, "The Histopathology Of Experimentally Infected Hamsters With The Lyme Disease Spirochete, Borrelia Burgdorferi", Proc. Soc. Exp. Biol. Med., 181, pp. 263-69 (1986); R. C. Johnson et al., "Infection Of Syrian Hamsters With Lyme Disease Spirochetes", J. Clin. Microbiol., 20, pp. 1099-101 (1984).]
Several animal models have been developed however, which suggest that it may be possible to immunize against B. burgdorferi infection. Early studies with hamsters showed that passive immunization, i.e. transfer of serum from rabbits inoculated with B. burgdorferi, conferred protection from subsequent infection with the same strain [R. C. Johnson et al., "Passive Immunization Of Hamsters Against Experimental Infection With The Lyme Disease Spirochete", Inf. Imm., 53, pp. 713-14 (1986)], however this immunity did not extend to strains from other geographic locations [R. C. Johnson et al. "Experimental Infection Of The Hamster With Borrelia Burgdorferi", Ann. N.Y. Acad. Sci., 539, pp. 258-63 (1988)]. In addition, active immunization of hamsters with whole inactivated B. burgdorferi also confers immunity, but again it appears to be somewhat strain specific [R. C. Johnson et al., "Active Immunization Of Hamsters Against Experimental Infection With Borrelia Burgdorferi", Inf. Imm. 54, pp. 897-98 (1986)]. Hamsters are not an optimal model system however, as they do not appear to develop the clinical symptoms associated with Lyme disease.
An animal model utilizing laboratory rats demonstrated that although they become persistently infected and develop arthritis and carditis, these symptoms are inconsistent if the rats are infected at 3 weeks of age or older [S. W. Barthold et al., supra.]
Another animal model system using the severe combined immunodeficiency (SCID) mouse has also been developed. SCID mice infected with B. burgdorferi contract a chronic infection associated with arthritis and carditis, similar to Lyme disease in humans. [U. E. Schaible et al., "The Severe Combined Immunodeficiency Mouse: A Laboratory Model For The Analysis Of Lyme Arthritis And Carditis", J. Exp. Med., 170, pp. 1427-32 (1989)]. Using this system, it was shown that B. burgdorferi-specific immune mouse sera as well as a monoclonal antibody to OspA, were able to prevent or slow the development of Lyme disease in SCID mice when passively transferred at the time of infection. [U. E. Schaible et al., "Monoclonal Antibodies Specific For The Outer Surface Protein A (OspA) Of Borrelia Burgdorferi Prevent Lyme Borreliosis In Severe Combined Immunodeficiency (SCID) Mice", Proc. Natl. Acad. Sci. USA, 87, pp. 3768-72 (1990)]. However, immunocompromised animals are not well suited for the study of potential vaccines. Others have attempted to infect immunocompetent strains of laboratory mice, but have failed, see S. W. Barthold et al., supra. Thus, additional animal systems and vaccine development is required.
As prevention of tick infestation is imperfect, and Lyme disease may be missed or misdiagnosed when it does appear, there exists an urgent need for the determination of the antigens of B. burgdorferi and related proteins which are able to elicit a protective immune response. In addition, in order to develop agents and methods to prevent and diagnose Lyme disease, an appropriate animal model which mimics the human disease is required with which to study and select such antigens, and to explore the immune response they may confer.